Video imaging systems have many different signal formats. Generally, each such video format has a reliable, unique set of frequencies for decoding and/or displaying the video signals. For example, most broadcast television video systems in the U.S. employ the NTSC format. Other countries use the PAL format
The NTSC format uses a frame frequency of 29.97 Hz (interlaced frames of video per second) while the PAL format uses 25 Hz. For 29.97 Hz NTSC, having 525 total scan lines having 486 visible scan lines, with a 4:3 aspect ratio, the clock frequency of the video signal is 13.5 MHz. For 25 Hz PAL, having 625 total scan lines, with a 4:3 aspect ratio, the clock frequency is 13.5 MHz.
Video recording playback and other broadcast equipment may also employ one of a number of different video signal formats. The video clock frequency for a video signal can be derived from synchronizing signals that accompany the video frames. Video signals have horizontal and vertical synchronizing (sync) signals or pulses that respectively indicate the end of a line of an image and the end of an image frame. In conventional systems which receive and process a video signal, a phase locked loop (PLL) system compares an incoming video signal to a reference signal and attempts to determine and lock to the appropriate video clock frequency and provide a stable output signal for use in processing the video signal. If an incoming video signal can be one of a plurality of possible video formats effort and time are required by the receiving circuits to determine the video format, so that processing circuitry can be suitably adjusted. As conventional systems attempt to determine and lock in on the appropriate clock frequency, the spectral purity of the resulting output clock signal which is used in the processing of the incoming video signal is impacted in that clock jitter occurs. Jitter is the oscillation of the output clock signal about a desired output frequency which occurs as the system searches for and locks onto the clock frequency of the incoming signal. There is a direct correlation between the possible range of input video signal clock frequencies, the range of the video signal output frequency and the jitter. The greater the frequency difference in a number of possible clock frequencies for incoming video formats which a receiving circuit must be able to lock to, the greater the likelihood and significance of clock jitter. Therefore, it is desirable to minimize the amount of output clock frequency jitter which occurs during detection and locking to the clock frequency of the incoming video signal.